A-Scan
A-scan ultrasound biometry, commonly referred to as an A-scan, is routine type of diagnostic test used in ophthalmology. The A-scan provides data on the length of the eye, which is a major determinant in common sight disorders. The most common use of the A-scan is to determine eye length for calculation of intraocular lens power. Briefly, the total refractive power of the emmetropic eye is approximately 60. Of this power, the cornea provides roughly 40 diopters, and the crystalline lens 20 diopters. When a cataract is removed, the lens is replaced by an artificial lens implant. By measuring both the length of the eye (A-scan) and the power of the cornea (keratometry), a simple formula can be used to calculate the power of the intraocular lens needed. There are several different formulas that can be used depending on the actual characteristics of the eye.
